The invention relates to an optical element made of a material that is transparent to wavelengths in the UV region, as well as to a projection lens and to a projection exposure apparatus for immersion lithography including such an optical element.
The wetting behaviour of surfaces of optical elements with liquids can be described by the so-called contact angle, which a droplet of liquid encloses with a surface of the optical element. If the liquid completely wets the surface, the contact angle is 0°; if the contact angle is less than 90° and water is the wetting liquid, this is referred to as a hydrophilic surface; and in the case of a contact angle of more than 90° as a hydrophobic surface.
Wetting with liquids, in particular with water, as a rule has a negative effect on the optical characteristics of the optical elements. As a result of wetting, it is possible, for example, for contaminating substances such as salts to form on the surfaces of the optical elements. In order to prevent wetting of optical elements with water or in order to quickly remove water from the optical elements, it is known to provide optical elements with hydrophobic coatings.
For example, in JP 2003-161806 A an optical element with an antireflection coating is described in which optical element a water-repellent coating is formed in a non-coated region of the glass substrate. In this arrangement the water-repellent coating can be formed so as to be adjacent to the antireflection coating, or on a lateral edge of the glass substrate. This is intended to prevent moisture lodging in the space between the optical element and a holding structure associated with the optical element.
In U.S. Pat. No. 5,494,712 a method for applying a layer of a polymer onto a substrate in order to reduce its wetting by water is described. The layer preferably comprises one or several organo-silicon compounds, e.g. silanes or siloxanes, and is applied by means of plasma-enhanced chemical vapour deposition (PECVD).
The problem of wetting also plays a role in microlithography, especially in immersion lithography. In microlithography, for the purpose of producing semiconductor components, structures on a mask are imaged at a reducing scale onto a light-sensitive substrate by means of a projection lens. In order to achieve the high resolution that is required for such applications, illumination radiation in the UV-wavelength region is used, typically below 250 nm. In order to achieve a further increase in the resolution and depth of field, in immersion lithography a liquid, as a rule distilled water, is placed between the last optical element of the projection lens and the light-sensitive substrate so as to increase the refractive index. In this arrangement the last optical element is at least in part wetted by water, which is why in the regions of the last optical element in which contact with water is to be avoided hydrophobic water-resistant coatings are used to an increased extent.
From JP 2005-268759 A a component has become known which is arranged in a projection exposure apparatus for immersion lithography, and which on at least one surface is subjected to exposure radiation. On the surface, an adhesive layer of silicon dioxide (SiO2), magnesium fluoride (MgF2) or calcium fluoride (CaF2) is arranged, onto which a water-repellent layer is applied that comprises an amorphous fluorocarbon polymer.
JP11-149812 describes an optical element in which a hydrophobic protective layer, preferably comprising a fluorocarbon polymer, is applied to a reflection-increasing or reflection-decreasing multilayer system so as to protect against ingress of humidity from the ambient air. The thickness of the protective layer is between 1 nm and 10 nm in order to prevent excessive absorption of the radiation by the fluorocarbon polymer at wavelengths of less than 250 nm.
In order to further increase the resolution and depth of field in immersion lithography, in future there will be immersion liquids that have a refractive index that is higher than that of water (nw=1.43). In the case of these highly refractive liquids, the solutions developed for water cannot, as a rule, be applied.